1. Field of the Invention
This invention relates to a switched-mode power supply, in particular a switched-mode power supply with a primary side and a secondary side, which has a transformer with a primary-side winding, a secondary-side winding and at least one auxiliary winding. The primary-side winding and the auxiliary winding are connected to the primary side and the secondary-side winding is connected to the secondary side. The switched-mode power supply comprises a primary-side switch, which is connected to the primary-side winding, in order to interrupt a current flow through the primary-side winding, a freely oscillating circuit for the generation of switching pulses, which drive the primary-side switch, and a circuit for the generation of an image voltage between the terminals of the auxiliary winding, in order to generate an image voltage, which on the primary side forms a voltage to be regulated on the secondary side.
2. Description of the Related Art
Switched-mode power supplies are used in numerous electronic devices to generate the low direct voltage required for the supply of the electronic components from a mains voltage. In this respect switched-mode power supplies have prevailed over conventional power supplies with mains transformers in many applications, because above a certain power class they exhibit a better efficiency and in particular require less space.
The latter is in particular attributable to the fact that instead of the mains voltage a high frequency alternating voltage is transformed, which, instead of the usual mains frequency of 50 Hz or 60 Hz, may for example be in the range from 20 kHz to 200 kHz. Since the required number of windings on the transformer falls inversely proportionally to the frequency, the copper losses can in this way be significantly reduced and the actual transformer becomes substantially smaller.
To further optimise the efficiency, in particular primary switched-mode power supplies are known in which the frequency generated on the primary side of the high frequency transformer by the switch, for example a bipolar transistor, is regulated in dependence of the load applied to the secondary side of the power supply unit in order to regulate the transferred power. The feedback required for this type of regulation is for example realised in that a voltage tapped off an auxiliary winding is used as the controlled variable. An appropriate method of controlling the output current and/or the output voltage is described in EP 1 146 630 A2 and takes into account that the same energy is loaded into the transformer with each pulse. However, the circuit arrangement shown in this document has the disadvantage of being of comparatively complicated construction, because a relatively complex integrated circuit is used as the control circuit.
The most inexpensive way of building a switched-mode power supply with electrical insulation between the primary and secondary sections is with a free-running flyback converter. This type of power supply however, has principally the disadvantage that with low load the switching frequency increases noticeably. Consequently, the power loss with no load and with low loads is high.
An indirect measurement of the output voltage by measuring the voltage on a primary auxiliary winding or the main primary winding is more difficult with this type of power supply. Due to the induced voltage from the stray inductance, a brief voltage overshoot arises, which with a large pulse width can be filtered out in a simple manner, so that it is possible to determine the secondary voltage relatively accurately. With a low load the pulse width however reduces so far that it is hardly possible to filter out the voltage induced by the stray inductance. This means that the output voltage on low load can only be determined very inaccurately. An example of this type of simple discrete circuit technology can be found in the (unexamined) published British patent application GB 02379036. In this circuit the use of an optocoupler is suggested to counter the disadvantages of unsatisfactory control accuracy. Such an optocoupler, however, increases in turn the complexity and the r costs of the complete switched-mode power supply.